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Black bears in the Sierra Nevadas typically "den" from December through early spring

But some emerge from hibernation early in what's been called a "disastrous winter"

Such bears live near places they can root through garbage; "wild bears" aren't affected yet

Official: It's also been unusually warm in Alaska, but bears there so far have stayed put

They barely slept.

That's the case for an unusual number of black bears in western Nevada, where an abnormally warm and dry winter has spurred them to halt their hibernation and head out for food.

"They may be sleeping under a deck or under a house in a crawlspace, and they will emerge when it is a garbage day and raid some garbage cans and then go back to a sleep for a while," explains Chris Healy, a spokesman for the Nevada Department of Wildlife.

Mother Nature can take credit for these bears' abbreviated slumber. Whereas parts of the Midwest and Northeast are now buried in snow and shivering in cold, that's not the case in many communities around Lake Tahoe and elsewhere.

It's not just Nevada: Nearby Northern California is also seeing an abnormal number of black bears, with CNN affiliate KXTV showing images taken from Yosemite National Park and Lake Tahoe's Heavenly Ski Resort.

Healy said "a disastrous winter" -- adding "we need the snow" -- and relatively mild temperatures have thrown off some black bears that live in more settled areas, where they can readily access human garbage.

"We documented this and, in the past 15 years, it occurs sporadically," the spokesman said. "A lot of it is dictated by the weather conditions."

Even in a typical winter, black bears don't hibernate completely as some might think, usually taking a few weeks for their metabolism and body temperature to wind down. They will occasionally wake up, roam a bit, but won't eat, drink, urinate or defecate, according to SierraWild.gov, a cooperative website of the federal U.S. Forest Service, National Park Service and Bureau of Land Management that covers the Sierra Nevada mountain range, including Lake Tahoe).

Bears in this range typically "den" between mid-December into March or the beginning of April, SierraWild.gov notes. This is less than the seven and a half months for some black bears that live in colder climes, but still well beyond what's being seen in parts of Nevada.

In the past, "lighter winters" have been correlated to "bigger bears that live among humans in Incline Village in the Lake Tahoe basin," notes Healy.

But it's not like every black bear is cutting short hibernation and heading out on the town.

"They eat nuts and berries and things that are not found in nature right now," he said, in contrast to the relatively ever-present trash rummaged through by other bears. "So they wouldn't wake up to eat, they wouldn't find anything."

It has also been "unusually warm" this winter in much of Alaska.

But state wildlife official Riley Woodford said that bears coming out of hibernation early hasn't been an issue there yet.

That's because the winter started off, around November, normally. Even though it's warmed up, the bears climbed up to 1,000 feet to den in snowy and icy spots that's cold enough to keep them in, he says.

Still, the state Department of Fish and Game admits that, hibernating or not, the weather has been weird. It's not even forecast to go below freezing -- even at night -- in Juneau for at least a week. And last week, it was even raining 3,000 feet above sea level in one ski resort around the Alaskan capital, Woodford recalls.